In general, hydrocarbo n compounds having in the molecule two or more carbon-carbon double bonds are called polyenes, a variety of which have been known, including, for example, 1,3-butadiene, 1,3-pentadiene, 1,4-hexadiene, ethylidene-2-norbornene (ENB), dicyclopentadiene and so on. By co-polymerizing such a diene with an .alpha.-olefin, such as ethylene, propylene or so on, an unsaturated copolymer having unsaturation bonds permitting vulcanization can be obtained. Due to its superior properties, such as weatharability, heat resistance and ozone-proof stability, such ethylenically unsaturated copolymer finds a wide variety of applications for, for example, automobile parts, industrial rubber products, electric insulator, articles for constructional and architectural uses and rubber products, such as rubber-lined cloth etc., and also for a material for polymer blends with, such as for example, polypropylene, polystyrene and others. Among these ethylene/.alpha.-olefin/diene copolymers, ethylene/propylene/5-ethylidene-2-norbornene copolymer exhibits, in particular, higher vulcanization velocity as compared with other ethylenically unsaturated copolymers and, hence, has especially widely been brought into practical uses.
Even this ethylene/propylene/5-ethylidene-2-norbornene copolymer, however, does exhibit inferior vulcanization velocity as compared with those commonly used diene-rubbers, such as natural rubber, styrene/butadiene rubber, isoprene rubber, butadiene rubber and nitrile rubber, and is inferior in the ability for co-vulcanizing with diene-rubbers.
Conventional ethylene/.alpha.-olefin/diene copolymers exhibit lower vulcanization velocity and, thus, are difficult to achieve an efficient productivity in producing vulcanized rubber products when reducing the vulcanization time or lowering the vulcanization temperature for the purpose of attaining a reduced energy consumption.
Therefore, it should be of great value in the industry, when such a polyene compound would be realized, that can afford to obtain, by co-polymerizing with an .alpha.-olefin, such as ethylene, an ethylenically unsaturated copolymer capable of being vulcanized at a high velocity and at the same time superior in the weatherability, heat resistance and fastness to ozone.
On the other hand, it has widely been recognized that, as a general property, a vulcanizable polymer exhibiting higher vulcanization velocity tends to have inferior scorch stability. This is believed due to that a polymer exhibiting higher vulcanization velocity may cause cross linking more easily during process steps other than the vulcanization, such as the compounded rubber storage step and rubber processing step before the vulcanization, as compared with polymers exhibiting lower vulcanization velocity, resulting in, thus, a premature vulcanization. Thus, a prompt vulcanizability is a reverse feature to the scorch stability, so that a polymer exhibiting both these properties may difficultly be obtainable. Thus, when it is attemped to increase the iodine value by increasing the proportion of ENB in the production of an ethylene/propylene/ENB copolymer, the vulcanization velocity may be increased, with a sacrifice in the scorch stability, resulting in a decrease in the workability on processing the rubber blend. Namely, troubles in the practical operation due to elevation in the rate of viscosity increase may apt to occur, by, for example, decrease in the extrusion output, increase in the motor load, stuffing of the cylinder or the die and so on. On the contrary, when a decrease in the iodine value is attempted, a decrease in the productivity of the vulcanized rubber due to retarded vulcanization may not be evaded, though an improvement in the scorch stability for the copolymer as a whole and, thus, in the workability thereof may be recognizable.